package 查找;
/**
 * 
* @ClassName: BinarySearch 
* @Description: TODO(二分查找,折半查找) 
* @author xiaok 
* @date 2018年10月5日 下午3:40:00 
*
 */
public class BinarySearch {

	/**
	 * 使用递归的二分查找
	 *title:recursionBinarySearch
	 *@param arr 有序数组
	 *@param key 待查找关键字
	 *@return 找到的位置
	 */
	public static int recursionBinarySearch(int[] arr,int key,int low,int high){
		
		if(key < arr[low] || key > arr[high] || low > high){
			return -1;				
		}
		
		//int middle = (low + high) / 2;			//初始中间位置
		int middle = low + ( high - low )/2;
		if(arr[middle] > key){
			//比关键字大则关键字在左区域
			return recursionBinarySearch(arr, key, low, middle - 1);
		}else if(arr[middle] < key){
			//比关键字小则关键字在右区域
			return recursionBinarySearch(arr, key, middle + 1, high);
		}else {
			return middle;
		}	
		
	}
	/**
	 * 不使用递归的二分查找
	 *title:commonBinarySearch
	 *@param arr
	 *@param key
	 *@return 关键字位置
	 */
	public static int commonBinarySearch(int[] arr,int key){
		int low = 0;
		int high = arr.length - 1;
		int middle = 0;			//定义middle
		
		if(key < arr[low] || key > arr[high] || low > high){
			return -1;				
		}
		
		while(low <= high){
			//middle = (low + high) / 2;
			 middle = low + ( high - low )/2;
			if(arr[middle] > key){
				//比关键字大则关键字在左区域
				high = middle - 1;
			}else if(arr[middle] < key){
				//比关键字小则关键字在右区域
				low = middle + 1;
			}else{
				return middle;
			}
		}
		
		return -1;		//最后仍然没有找到，则返回-1
	}

	public static void main(String[] args) {
		 
		int[] arr = {1,3,5,7,9,11,18,111,222,666,7777,123131};
		int key = 123131;
		//int position = recursionBinarySearch(arr,key,0,arr.length - 1);
		
		int position = commonBinarySearch(arr, key);
 
              if(position == -1){
			System.out.println("查找的是"+key+",序列中没有该数！");
		}else{
			System.out.println("查找的是"+key+",找到位置为："+position);
		}
		
	}

	
}
